Well stimulation

ABSTRACT

Methods of conducting matrix acidizing in a well in a formation by simultaneously or alternatingly injecting an acid and a diverter into the formation through tubing. The acid is injected into the formation through a first tubing and the diverter is injected into the formation through a second tubing. The second tubing may extend within said first tubing and the formation may be a chalk formation.

A well for the production of hydrocarbons may be stimulated by one or more procedures. The purpose of stimulation is to increase the permeability to the flow of hydrocarbons. These procedures may increase the permeability to values not only equal to the original natural permeability prior to drilling. If a hydrocarbon reservoir has acceptable saturation, but insufficient permeability for economic development, stimulation procedures may also increase the permeability to a value justifying the cost for development of the reservoir.

Matrix acidizing is a common way to stimulate a well by pumping acid into the near wellbore region to dissolve formation damage and create pathways for the hydrocarbons.

Various acids may be used to Improve production by dissolving formation damage or creating new pathways around the borehole depending on the nature of the formation, rock, or the types of damage which may exist. In acidizing a well, an acid solution Is pumped under pressure down the well and into the perforations around the well, chemically removing contaminants as well as rock structure itself. However, if the pressure exceeds the fracture closure pressure (FCP), pumping acid into the matrix of the formation may cause a hydraulic fracture of the formation and create an undesirable flow path between wells. It Is therefore essential to keep the pressure and rates of the fluid below fracture closure pressure (FCP). The FCP is a function of formation parameters and pressure.

Low acid pressure injection rates often cause other problems, because when acid is injected at low rates the most reactive parts of the formation in a group of perforations or zones may take up all the acid. With continued reaction at low injection rates it is even more likely that only these more reactive perforations or zones will receive acid.

As acid is pumped it preferably flows along the most permeable path into the formation. The acid opens up these paths even more, and the process becomes self-perpetuating; and less permeable, damaged zones are almost guaranteed not to receive adequate treatment. Some technique to divert the treatment fluid is therefore essential. Such technique is called diversion.

There exist a variety of diversion techniques. Treatment fluid can be directed exclusively towards a low-permeable zone using mechanical aids or the flow itself can be blocked at individual perforations using e.g. ball sealers that seal the perforation. In order to cover all zones as completely as possible, a frequent practice is to use diverting materials in combination with stages of acid.

The most common materials used to divert the acid are particulates that are poorly soluble in acid, but soluble in hydrocarbons for effective cleanup so as not to obstruct the production.

Because of both Its solubility in hydrocarbons and its capability to build up filter cakes within the holes created by the acid, and thereby forcing the acid to divert, stimulation is often performed with benzoic acid flakes, emulsified acids, oil soluble resins, foam and self-viscosifying acids as a diverting material.

One method is to use chemical diversion during the matrix stimulation treatment and this can be accomplished by adding diversion material in stages at the surface such that the diversion material is separated by stages of acid and pumping the stages of acid and diversion material commingled through the tubing without mixing. These stimulation treatments are normally pumped at high rates and at high treatment pressures and therefore they are called High Rate Matrix stimulation methods. The diversion volumes are large compared to the total treatment volumes and the diverting materials are physically very coarse. Therefore, the High Rate Matrix stimulation methods often cause severe operational problems, such as stuck pipes or tubing, or the FCP is in many cases even exceeded during the treatment causing the formation to fracture.

These problems can be partly avoided by using a coiled tubing which is basically a second smaller tubing disposed within the production tubing (first tubing) running from the well to the surface. Coiled tubing is widely used in the oil and gas industry for completion, production and work-over operations. Some of the oilfield operations in which such a secondary tubing is used are completion operations in which the second tubing can be used to transport fluids from the surface down into the well. The treatment fluids in the prior art technique involving coiled tubing are pumped through the interior of the second tubing and into the well to perform the particular required operation which can take place at a predetermined location or in a predetermined depth. The second tubing may remain suspended in the well for continuous use, and it may extend from the surface to the bottom of the well. The second tubing may also extend to an intermediate point between the surface and the well bottom. The second tubing may possibly also be temporarily suspended into the well for the duration of a particular operation during which the tubing may be raised and/or lowered to various levels in the well. Upon completion of the operation, the tubing may be retracted so as to be used in another operation or be transported to another well.

As will be familiar to those having operating experience with well stimulation by matrix acidizing, the prior art technique often poses a potential hazard to the environment and pollution is often the unsuccessful outcome in situations where the formation is blocked with diversion materials and the treatment pressure at the same time approaches or even exceeds the FCP. The tubing is then filled with a mixture of acid and diversion material, and further pumping is not possible without unavoidable fracturing of the formation. The tubing content of acid and diversion material (possibly solid) then inevitably have to be circulated out back to surface and discharged to the sea where both the acid and the diversion material present a high degree of hazard to the environment.

It is an object of the invention to provide a method for diverting acid into all intervals of a formation via a diverting agent, which method further minimizes chemical consumption and prevents discharge of acid and diverting material to the sea.

This is achieved by the present invention as defined by the claims relating to methods of conducting matrix acidizing in a well in a formation by simultaneously or alternatingly injecting an acid and a diverter into the formation through tubing, said acid being injected into the formation through a first tubing and said diverter being injected into the formation through a second tubing. The second tubing may extend within said first tubing and the formation may be a chalk formation.

Having acid and diverter in both the first and the second tubing respectively makes it possible to inject one of these fluids at the precise moment where the particular treatment fluid is needed in the matrix acidizing process thus improving the utilization of well treatment fluids.

It is a further object of this invention to increase the pressure during acidizing and thereby to improve permeability of a the formation and stimulate said formation to produce increased volumes of hydrocarbons.

Increasing the pressure at which acid is injected into the formation improves the matrix stimulation of a well since the higher pressure forces the acid further into the formation. Accordingly an even better exploitation of especially the acid can be accomplished by monitoring of the pressure e.g. at the wellbore or formation to facilitate an almost instantaneous injection of acid whenever the pressure is approaching FCP pressure or/and injection of diverter whenever the pressure decreases to a level inadequate for the matrix acidizing of the well.

In a preferred embodiment the formation is pre-flushed by acid that is injected into the formation through-said second tubing.

Preferably, the acid treatment is initiated through the second tubing, whilst taking fluids back to surface via the first tubing inlet until the acid reaches the end of the second tubing. The initial step of Injecting a relatively small amount of acid into the formation through the second tubing causes the formation pressure to decrease. This will improve the injectivity into the formation so the annular volume (between the first and the second tubing) of non-reactive fluid can be injected to the formation, without exceeding the FCP. Advantageously the injection occurs by displacement of the annular volume by well treatment fluid. However the substitution of said liquid could also, in a less preferred embodiment, be accomplished by it being displaced through the top from the bottom end (at the formation) with the aid of the inner (second) tubing.

The use of benzoic flakes dissolved in methanol achieves a highly flowable diverter that can flow into all parts of a formation and provide good diversion without exhibiting defects due to segregation in the tubing delivering it.

The formation being a chalk formation.

The invention will be described in detail in the following with reference to the drawing in which

FIG. 1 shows a sectional view of a well showing the initial step of displacing fluid in second tubing by acid whilst taking fluid back to surface through first tubing inlet,

FIG. 2 shows a well being pre-flushed with acid through the second tubing,

FIG. 3 shows a well with diverter in the second tubing,

FIG. 4 shows a sectional view of a well showing the step of injecting the annular volume of non-reactive fluid (between first and second tubing) into formation by displacement with acid,

FIG. 5 shows a sectional view of well showing displacement of fluid in the second tubing by diverter,

FIG. 6 shows a sectional view of a well having diverter present in the second tubing and acid present in the first tubing at the formation,

FIG. 7 shows a sectional view of a well where the acid is being displaced by a non-reactive fluid,

FIG. 8 shows a sectional view of a well where the acid is being displaced by a non reacting fluid, and

FIG. 9 shows a sectional view of a prior art well with a single tubing having acid and diverter commingled through the tubing.

Referring more particularly to the drawings, FIG. 1 discloses a well having a casing 3 extending throughout the wellbore, which is usually secured in place by e.g cement. The casing 3 is perforated 5 adjacent to the production formation 20 to provide flow passages for fluids from the formation 20 into the casing 3. A first tubing or production tubing I has a sliding door 6 and a tubing Inlet 12 extends into the casing 3 and has a packer 19 (already known in the prior art) for isolating the production interval of the well-bore. A second tubing 2 extends down within the production tubing 1 into the well-bore and has an inlet 11. The second tubing, which is commonly called coiled tubing, may be wound around a reel or wheel and Is passed into the well through an—already known—injector head.

The second tubing and production tubing further comprises a system of valves (not shown) having means for individually and independently regulating and turning off the fluid flow through first and second tubings at both inlet and welbore end. The second tubing usually passes through a blowout-preventer (not shown either) to facilitate regulation of the well-bore pressure. The coiled tubing usually consists of a singular tubing but may consist of two or more tubings.

The outcome of matrix acidizing depends highly on the pressure at which diverter and acid are injected and the aim of the current matrix stimulation method is to deploy acid at the highest pressure possible without exceeding of the pressure at which the formation breaks down and a fracture is generated. The pressure at which the formation breaks down is called the fracture closure pressure (FCP). To facilitate a more precise pressure in the wellbore, the pressure may be monitored. Monitoring of the wellbore pressure facilitates injection of acid whenever the pressure is adequate for acidizing and diverter whenever the pressure drops to a level inadequate for acidizing thus optimizing utilization of the stimulation fluids.

When initiating the stimulation of a well as shown in FIG. 1, the non-reactive liquid, present in the second 2 (coiled) tubing is displaced by acid (illustrated by being a little darker in FIG. 1). The acid 8, which e.g. can be a 15% HCl, is then pumped in from the top of the second tubing 2 whilst taking fluid back via the first tubing 1, thus essentially maintaining formation pressure at the wellbore.

The tubing 1 is then closed and the acid treatment is initiated with a pre-flush of acid (bull-heading) to the formation via the second tubing as shown in FIG. 2. The purpose of initially injecting of a pre-flush of acid 8 through the second tubing is to open up the formation so as to increase its injectivity sufficiently to absorb the volume of non-reactive fluid present in the first tubing 1 (production tubing) without exceeding the fracture propagating pressure of the formation. Field testing shows that this is advantageously done by injecting approximately 50 bbl (barrel) of acid. The acid pre-flush is advantageously finished by displacement of acid in the second tubing by diverter 9 as shown in FIG. 3.

Preferably, the main acid treatment is then started by injection of the (annular) volume of non-reactive liquid present in the first tubing 1 into the formation by displacement with acid 8 as shown in FIG. 4. The flow is kept at rate such that the pressure does not exceed the FCP pressure. The acid injection preferably continues until the pressure starts to decrease. However, a pressure insufficient for acidizing may at any time during the well stimulation procedure cause injection of diverter 9.

When the main treatment acid 8 starts to react with the formation, FIG. 5 shown by a pressure drop, a batch of diverter is pumped into the main treatment fluid and further to the formation via the second tubing (coiled tubing), as shown in FIG. 6. The diverter will follow the main stream, which is also the path of least restriction, into the formation and preferably block off the coming acid accession to recently stimulated areas in the formation.

When the path into the formation is blocked by diverter 9, the treatment pressure will increase thus indicating that acid 8 would be diverted to other parts of the formation. When the pressure has increased to a level adequate for acid treatment but preferably still below FCP, acid 8 is injected via the first tubing. This treatment is then continued until pressure has again dropped to a level sufficiently below FCP-pressure. The main treatment acid 8 is displaced to perforations, as shown in FIG. 7 and 8.

The worst operational scenario during prior art methods of matrix stimulation with diverter 9 is illustrated in FIG. 9. The FCP is exceeded and further pumping into the formation will invariably create fracture of the formation. Further, the string content has to be circulated back to surface and disposed off.

The acid utilized may be any of the aqueous solutions of acid commonly employed for acidizing formations. The solution of acid may be an aqueous solution of hydrochloric acid and hydroflouric acid, which is employed for acidizing formations. Certain organic acids can be utilized alone or in combination with inorganic acids. Organic acids Include formic, acetic and oxalic acids. When hydrochloric acid Is utilized, it should be in a concentration from about 5 to about 30 percent. When combined with hydrochloric acid, the concentration of hydroflouric acid will range from about 0.3 to about 8 percent.

Although the method has been described as comprising diverter in the second tubing and acid in the first tubing it is clear that the opposite condition may as well be the case without departing from the scope of the invention. 

1. A method of conducting matrix acidizing in a well in a formation by simultaneously or alternatingly injecting an acid and a diverter into the formation through tubing, said acid being injected into the formation through a first tubing and said diverter being injected into the formation through a second tubing.
 2. A method according to claim 1, wherein before said acid and said diverter are injected alternatingly through the first and the second tubings respectively, the formation is pre-flushed by acid that is injected into the formation through said second tubing.
 3. A method of conducting matrix acidizing as defined in claim 1, wherein said diverter comprises benzoic acid diluted in methanol.
 4. A method of conducting matrix acidizing as defined in claim 1, wherein said formation is a chalk formation.
 5. A method of conducting matrix acidizing in a well in a formation by simultaneously or alternatingly injecting an acid and a diverter into the formation through tubing, said acid being injected into the formation through a first tubing and said diverter being injected into the formation through a second tubing, said second tubing extending within said first tubing.
 6. A method according to claim 5, wherein before said acid and said diverter are injected alternatingly through the first and the second tubings respectively, the formation is pre-flushed by acid that is injected into the formation through said second tubing.
 7. A method of conducting matrix acidizing as defined in claim 5, wherein said diverter comprises benzoic acid diluted in methanol.
 8. A method of conducting matrix acidizing as defined in claim 4, wherein said formation is a chalk formation. 